1. Field of the Invention
This invention relates to the production of uniform sub-micron size ceramic powders suitable for sintering after pressing or casting to give solid ceramic bodies of near-theoretical density. The invention is particularly applicable to the production of such powders of pure zirconia and of zirconia doped with oxides such as yttria, other rare earth oxides, magnesia, lime, and combinations of any of these, which are known to stabilize the cubic crystal form of zirconia, but it is also applicable to production of sub-micron sized powders of alumina, silica, titania, and mixtures of these with a variety of other oxides.
2. Description of the Prior Art
The most common method for the production of sub-micron sized powders of the ceramic oxides is precipitation or co-precipitation of an oxide precursor from aqueous solution of a salt of the metal or metals to be used. Commonly, oxides, hydroxides, oxycarbonates, oxalates, etc., all usually with varying degrees of hydration, are precipitated at an appropriate pH. In all these cases, the precipitate normally has the form of a gel, which presents serious practical processing problems: First, filtration is very difficult because of clogging of the pores of the filter. Second, desalting of the gel by washing is very tedious as a result of the filtration problem; moreover, gradual peptization of the gel to sol results from the gradual drop in electrolyte concentration induced by washing and worsens the filtration problem still further. Third, oven drying of the purified gel often yields unsinterable powders.
Other established prior art method for the production of sinterable zirconia powders include high temperature oxidation of zirconium chloride with oxygen and/or water vapor, hydrolysis or thermal decomposition of zirconium alkoxides, and hydrothermal reaction of zirconium metal or its alloys. These methods, however, require complex and expensive equipment and/or expensive starting materials and thus find relatively little industrial use.
The recent U.S. Pat. No. 4,365,011 to Bernard et al. discloses an alternative precipitation method for the preparation of sinterable zirconia powders. According to the Bernard teaching, a solution of appropriate metal salt precursors is formed in alcohol rather than in water. A precipitate is formed by the action of aqueous ammonia on such a solution; the precipitate is washed with a hydrophilic solvent to eliminate most residual water, dried, and calcined. The drying step is preferably accomplished by azeotropic distillation with a third solvent such as benzene which is capable of displacing both water and the alcohol used. From this and other indications in the Bernard specification, I have concluded that the precipitate formed according to its teachings is a gel, and therefore is subject to all the processing difficulties for gels described above.
In my copending application Ser. No. 06/511,058, filed July 25, 1983, I described how sub-micron sinterable zirconia and other oxide powders could be synthesized in a non-aqueous solvent such as ethanol. A substantial advantage of that process is that no gel is formed, so that coarsening of the powder during drying and calcination is avoided. A disadvantage of that method is the requirement for large amounts of relatively expensive organic solvent.